The invention relates to a method of producing a compound material of chromium and copper which is used as contact material for medium voltage vacuum power switches.
A Cr Cu compound with about 40 to 60% Cr has been proven successful as a contact material for vacuum power switches. The Cu component ensures sufficient electrical and thermal conductivity, while the skeleton material Cr diminishes burnoff and also, having, in comparision with tungsten, a low melting point of about 2173 degrees K., eliminates the danger of harmful thermal electron emission. In addition, the Cr greatly reduces the tendency of the contact pieces to weld together and it also possesses good getter properties.
Because of the miscibility gap in the system Cr-Cu, only powder metallurgical methods can be considered for the production of the compound material CrCu for the desired concentration range of about 40 to 60% Cr. The most commonly used method involves the making of compacts of Cr powder or CrCu powder mixtures, the pores of which are filled with liquid Cu after the sintering. However, such sintering-impregnating methods as well as the other powder metallurgical methods are difficult to control because of the tendency of the chromium to oxidize. In particular there is danger that pore or impregnation defects will result due to poor wettability of certain grain surfaces or due to the formation of passive layers. Even if these defects are of the order of only 5 to 50 microns, they can impair the switching properties. In the practice this results in a certain scatter in circuit breaking performance.
In other existing methods, porous blanks, for example, are produced by the pressing or pouring of metal power. These blanks either consist of pure Cr powder or are admixed with one or more additional powder additives to obtain a liquid phase during sintering. The subsequent sintering under high vacuum or in pure shielding gas at temperatures of 1573 degrees K. to 1773 degrees K. leads to a desired formation of sinter bridges between the powder grains. This increases the skeleton stability and permitts safe handling of the porous sintered blanks after the sintering process. In the next step the blanks are placed in impregnation molds or on impregnation substrates, are given as application or backing an amount of impregnation metal, such as copper, corresponding to the pore volume, and are again heated under high vacuum or in pure shielding gas above the melting temperature of the impregnation metal. This causes as infiltration of the porous skeleton to occur due to capillary forces.
The above described impregnation method for the production of the Cr-Cu compound materials, does not produce completely faultless impregnations despite very careful working procedures. This is due essentially to three reasons:
When recharging the furnaces between the sintering and impregnating processes, the skeleton surface of the highly getter-active Cr skeletons will acquire a new covering of thin oxide films or chemisorbed gas films which make wetting with the liquid impregnation metal difficult. For thermodynamic reasons these oxidation processes occur below about 1000 degrees K. even under high vacuum and in pure shielding gas, because oxygen partial pressures below 10.sup.-10 mb cannot be obtained in economically feasible furnaces. As a result of this phenomenon, impregnation defects occur which manifest themselves in the form of micro-cavities and pores.
As a result of the sintering process and the formation of sinter bridges connected therewith, inaccessible pore regions are obtained which are only partially reached or not reached at all by liquid impregnation metal. Also the possibility of bringing reducing substances, such as carbon, to the skeleton metal via the liquid impregnating metal phase is limited, so that in these residual pore regions caused by sinter bridge formation, there are residual oxides which impair the switching capacity of the material.
The stiffening effect of solid sinter bridges considerably reduces the deformability of the skeleton, material. As the Cr skeleton which is impregnated with copper or copper alloys, is cooled from the infiltration temperature of the liquid impregnating metal, there occurs, because of the different thermal expansion between Cr and Cu, a volume deficit which cannot be absorbed by joint uniform shrinkage of the skeleton and impregnating metal. This phenomenon can also lead to lattice defects and to microporosities, invisible under a light-optical microscope, which may lower the quality of the material for high-power switching application.
Attempts have been made to avoid these problems. For instance, Cr powder and Cu powder may be mixed, thereby suppressing direct contact of the Cr grains to a large extent, and in the subsequent sintering process only sporadic deformation-inhibiting sinter bridges will form; or in some cases no bridges will form. Although this production process removes the steric impediment of the Cr particles, sufficient switching performance cannot be achieved with such a material. This is due to the interaction between the Cu powder, normally contaminated with about 500 ppm oxygen, and the getter-active Cr powder. Already below 1273 degrees K., i.e. 1000.degree. C., as Cu.sub.2 O dissociation sets in, the oxidation-avid Cr powder is oxidized to a higher degree. Because of the high heat of oxidation of Cr, stable surface oxides will form which cannot be removed by normal vacuum degasing.